From a product performance perspective, technological progress of disk drives can be measured with three metrics; storage capacity, peak data transfer rate, and head seek time. As an example, considering one major disk drive family over a nine-year period, capacity increased 32 times, maximum transfer rate increased eight times, but full-head seek time decreased 1.6 times. As a result, the disparity between capacity improvements and head seek times has led to a degradation to overall system performance.
Some system designs have attempted to improve storage response times by adding cache memory inside disk drives. However, adding hardware increases system cost and also requires additional functionality to properly manage the cache memory. Other designs take advantage of regional placement of data near a disk's perimeter. Data placed near the perimeter of a disk drive platter has faster transfer rates than data placed near the hub of the disk drive. However, disk throughput improvements from intelligent data placement often degrade with changes in workload.